1. Field of the Invention
The present invention relates to a process for the preparation of an indole from aniline or a derivative thereof and a polyhydric alcohol, and more specifically to a preparation process which makes use of a supported silver catalyst.
2. Description of the Related Art
In U.S. Pat. No. 4,476,310, the present inventors have already proposed a process for preparing an indole by reacting aniline or a derivative thereof with 1,2-glycol in the presence of a silver catalyst supported on a carrier whose specific surface area is 10 m.sup.2 /g or greater. As is also pointed out in U.S. Pat. No. 4,831,158, when the reaction is conducted using such a catalyst, the catalyst is progressively deteriorated as the reaction time goes on. As a result, the conversion of the glycol and the selectivity to indole or the indole derivative are lowered, whereby the yield of indole or the derivative thereof is reduced. The thus deteriorated catalyst can be regenerated by the method disclosed in U.S. Pat. No. 4,831,158, thereby making it possible to resume the reaction. However, the silver catalyst supported on the carrier can retain its activity only for a short time, leading to the need for frequent regeneration. Further, a significant amount of time is required for the regeneration so that the operation rate is impaired. Irreversible deactivation of the catalyst is also unavoidable upon regeneration. There has hence been a strong desire for the development of a catalyst whose catalytic performance undergoes minimized deterioration during both the reaction and the regeneration.